The main structure of the rail bridge being lifted
into place during a night-time possession of the railway
line Carmarthenshire County Council commissioned Mott
MacDonald to design a £7 million link road between the Morfa
Crossing Roundabout and Berwick Park Roundabout in south east
Llanelli. The scheme has involved the construction of 2.6km of S2
highway over the main South Wales to London railway plus several
watercourses.
The site comprised highly compressible ground conditions within
a tidal flood plain and necessitated detailed investigation of
ground conditions and the design of environmental mitigation
measures. This is one of the first assignments to be procured using
ECC option C with early contractor involvement to assist in the
provision of economic design and construction.
Embankment construction leading up to the railway
crossing over the South Wales Mainline Road
embankment design
Road embankments were required of up to 9m in height, constructed
over weak and compressible conditions comprising 15m of estuarine
and marine deposits. These are predominantly cohesive and include
significant layers of peat. Embankment designs included surcharge
embankments and also embankments founded on piles and, vibro stone
column load transfer platforms. Reinforced soil slopes and
reinforced soil walls were used to form the approaches to the rail
bridge, while finite element analysis was used to optimise the
design and predict ground movements on an adjacent rail line.
Settlement
An observational method was carried out for construction, whereby
the embankments were monitored by a suite of ground instruments
during construction.
Settlement plotThis approach involved
construction of the embankments carried out in stages allowing
'wait periods' when the embankments are at part height to permit
consolidation of the underlying ground. This system combined with a
robust monitoring, back analysis and reporting regime helped ensure
that embankment stability is maintained at an appropriately high
level throughout construction.
Construction of the rail bridge
Management of ground movement associated with rail
lines
The alignment of the new link road passes over and adjacent to two
separate operational railway lines. Ground movement affecting the
track alignment is of significant concern, therefore close
co-operation was required with Network Rail to produce an
acceptable framework based upon which to manage the risk of
potential ground movement.
Rail bridge
The rail bridge is formed with a multiplate corrugated steel arch
on piled foundations. Spandrel and wingwalls are formed in
reinforced earth retaining walls. The structure is over 52m in
length with a span of 15.5m maikng it the largest bridge of its
kind in the world.
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